1. Field of the Invention
This invention relates to a battery-powered DC to DC converter for charging an energy-storage capacitor. More particularly, this invention relates to a DC to DC converter which is operated dynamically as a function of battery condition so that an optimum compromise is made between the performance of the battery and the performance of the converter in an application such as charging an electronic strobe flash unit capacitor.
2. Description of the Prior Art
The electrical energy a battery delivers to a load equals battery power (battery voltage times battery current) multiplied by the time during which that power is delivered. A battery has a certain energy-delivery capacity. This capacity is not constant, and is a function of battery loading and other factors such as temperature. If battery voltage is halved and battery current is doubled, the battery's total energy-delivered capacity can change, even though battery power is the same. An excessively loaded battery, i.e. a battery whose current is increased beyond a particular level, can suffer a significant decrease in its total energy-delivery capacity.
As the condition of the battery changes due, for example, to aging or to changes in environment, the peak current that it can supply, without being excessively loaded, changes. It is desirable, therefore, that the battery load be adjusted in accordance with battery condition so that an optimum compromise is made between the performance of the battery and the performance of its load.
In a battery-powered electronic strobe flash unit, a flash-firing capacitor is charged to a voltage of approximately 300 volts so that the strobe can be fired. To charge the capacitor to this level using a low-voltage battery, the strobe unit includes a DC to DC converter. For a 6-volt battery, a conventional DC to DC converter for an electronic strobe flash unit can require a peak current greater than an ampere for a fraction of a second and an average current of about 750 milliamperes for 10 seconds or more.
There are two somewhat conflicting requirements that govern the construction and operation of a DC to DC converter for an electronic strobe flash unit. Firstly, it is desirable that the converter operate to charge the flash-firing capacitor quickly. This permits a camera operator to take a rapid sequence of flash exposures. However, if the capacitor is charged as quickly as possible the battery can be overloaded, thereby reducing its total energy-delivery capacity.
Secondly, it is desirable to transfer electric energy from the battery to the flash-firing capacitor efficiently. This permits a greater number of flashes to be produced before the battery is exhausted. However, if the converter is operated solely on the basis of high power-transfer efficiency, the transfer of power from the battery to the capacitor can occur at a slow rate, so that the time required to charge the capacitor can be excessive.
U.S. Pat. No. 4,104,714, issued to Smith et al on Aug. 1, 1978, describes a DC to DC "flyback" converter that is intended to operate so that maximum power is transferred from the battery to an energy-storage capacitor. The flyback converter includes a first current-sensing circuit for sensing battery current to a transformer, and for interrupting the battery current when it reaches a predetermined magnitude. The transformer stores energy when battery current is supplied to the transformer, and the transformer delivers its stored energy to the energy-storage capacitor in the form of a charging current when battery current to the transformer is interrupted. A second current-sensing circuit monitors the current charging the capacitor, and inhibits further battery current to the transformer until the energy stored in the transformer has been delivered to the storage capacitor. A battery voltage-sensing circuit then continues the interruption of the battery current if the battery open-circuit voltage has not recovered to a predetermined level.
An arrangement of the type described in the '714 patent can require an excessive amount of time to charge the storage capacitor. For example, as the battery ages, its open-circuit voltage may not recover quickly to the aforementioned predetermined level. As a result, the duty cycle of the converter can decrease to a point where the capacitor charging time is unacceptably long. Furthermore, if the internal resistance by the battery increases as the battery's condition changes, the battery may not be able to supply a current of a magnitude that causes the first current-sensing circuit to interrupt current flow. In this case, the converter will attempt to draw energy from the battery that it cannot supply, thereby possibly permanently damaging the battery.
U.S. Pat. No. 4,070,699, issued to H. M. Einbinder on Jan. 24, 1978, describes a flyback converter that is intended both to minimize capacitor charging time and to provide optimum battery use. The converter includes control circuitry that allows only minor changes in the magnetic field of the transformer, both when the battery current flows and when it is interrupted. With this arrangement, the battery current is maintained approximately constant whenever it flows.
A converter of the type described in the '699 patent must operate at a high frequency for the battery current to approach a constant. As a result, frequency-dependent factors such as transition losses in the transformer core reduce power transfer efficiency. Furthermore, the control circuitry does not operate the converter dynamically, i.e. it does not compensate for changes in the condition of the battery.